Ascaris lumbricoides

style="background:#Template:Taxobox colour;"|Ascaris lumbricoides
An adult female Ascaris worm
style="background:#Template:Taxobox colour;" | Scientific classification
Kingdom: Animalia Phylum: Nematoda Class: Secernentea Order: Ascaridida Family: Ascarididae Genus: Ascaris Species: A. lumbricoides
Binomial name
Ascaris lumbricoides Linnaeus, 1758

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This page is about microbiologic aspects of the organism(s).  For clinical aspects of the disease, see Ascariasis.

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]}}; Associate Editor-In-Chief: Imtiaz Ahmed Wani

Overview

Ascaris lumbricoides is the giant roundworm of humans, growing to a length of up to 35 cm (13.779527545 in). It is one of several species of Ascaris. An ascarid nematode of the phylum Nematoda, it is the largest and most common parasitic worm in humans. This organism is responsible for the disease ascariasis, a type of helminthiasis and one of the group of neglected tropical diseases. An estimated one-sixth of the human population is infected by A. lumbricoides or another roundworm. Ascariasis is prevalent worldwide, especially in tropical and subtropical countries.

Lifecycle

File:Ascaris lumbricoides life cycle.tif

Image showing lifecycle inside and outside of the human body of one fairly well described helminth: A. lumbricoides

A. lumbricoides, a roundworm, infects humans when an ingested fertilised egg becomes a larval worm that penetrates the wall of the duodenum and enters the blood stream. From there, it is carried to the liver and heart, and enters pulmonary circulation to break free in the alveoli, where it grows and molts. In three weeks, the larva passes from the respiratory system to be coughed up, swallowed, and thus returned to the small intestine, where it matures to an adult male or female worm. Fertilization can now occur and the female produces as many as 200,000 eggs per day for a year. These fertilized eggs become infectious after two weeks in soil; they can persist in soil for 10 years or more.

The eggs have a lipid layer which makes them resistant to the effects of acids and alkalis, as well as other chemicals. This resilience helps to explain why this nematode is such a ubiquitous parasite.

Morphology

File:Ascaris lumbricoides4.jpg

Fertile egg as can be seen in a microscope

File:Ascaris lumbricoides.jpg

Fertile egg in human faeces (detail)

File:Ascaris lumbricoides non-fertile egg.jpg

Infertile egg

A. lumbricoides is characterized by its great size. The male's posterior end is curved ventrally and has a bluntly pointed tail. Females are wide and long. The vulva is located in the anterior end and accounts for about one-third of its body length. Uteri may contain up to 27 million eggs at a time, with 200,000 being laid per day. Fertilized eggs are oval to round in shape and are long and wide with a thick outer shell. Unfertilized eggs measure long and wide.

Genus and Species	Ascaris lumbricoides
Common Name	Giant Intestinal Roundworm
Etiologic Agent of:	Ascariasis
Infective stage	Embryonated Egg
Definitive Host	Man
Portal of Entry	Mouth
Mode of Transmission	Ingestion of Embryonated egg through contaminated food or water
Habitat	Small Intestine
Pathogenic Stage	Adult Larva
Mode of Attachment	Retention in the mucosal folds using pressure
Mode of Nutrition	Feeding of Chyme
Pathogenesis	Larva – pneumonitis, Loffler’s syndrome; Goes through a Blood-Lung Phase (Hookworm and Strongyloides stercoralis also have a blood-lung phase); Adult worm– Obstruction, Liver abscess, Appendicitis.
Laboratory diagnosis	Direct Fecal Smear; Concentration methods such as Kato-Katz
Treatment	Albendazole, Mebendazole, or Pyrantel Pamoate
Diagnostic Feature - Adult	Female - prominent genital girdle
Diagnostic Feature - Egg	Coarse mammilated albuminous coating

Gallery

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  Magnified 128X, this photomicrograph revealed some of the ultrastructural features displayed by a fertile Ascaris lumbricoides egg. A. lumbricoides is the largest nematode (roundworm) parasitizing the human intestine. From Public Health Image Library (PHIL).

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  Magnified 125X, this photomicrograph revealed the presence of a fertile Ascaris sp. egg that was found in an unstained formalin-preserved stool sample. See PHIL 411 for an example of an unfertilized Ascaris lumbricoides egg. From Public Health Image Library (PHIL).

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  Depicted in this 1960 photograph were two Ascaris lumbricoides nematods, i.e., roundworms. The larger of the two was the female of the species, while the normally smaller male was on the right. Adult female worms can grow over 12 inches in length. From Public Health Image Library (PHIL).

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  This micrograph reveals both fertilized (A) and unfertilized (B) Ascaris eggs, and a Trichuris egg (C); Mag. 125X. From Public Health Image Library (PHIL).

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  This diagram depicts the various stages in the life cycle of the intestinal roundworm nematode Ascaris lumbricoides. From Public Health Image Library (PHIL).

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  These are 3 fertilized A. lumbricoides eggs with the one on the right being decorticated, for its outer layer is absent. From Public Health Image Library (PHIL).

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  This micrograph depicts an embryonated Ascaris lumbricoides egg with no outer mammillated layer, i.e., “decorticated”. From Public Health Image Library (PHIL).

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  This photomicrograph depicts a fertilized egg of the parasite Ascaris lumbricoides. From Public Health Image Library (PHIL).

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  Magnified 128X, this photomicrograph revealed some of the ultrastructural features displayed by an infertile, decorticated Ascaris lumbricoides egg. From Public Health Image Library (PHIL).

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  Magnified 128X, this photomicrograph revealed some of the ultrastructural features displayed by an infertile, Ascaris lumbricoides egg. From Public Health Image Library (PHIL).

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  Magnified 128X, this photomicrograph revealed some of the ultrastructural features displayed by a fertilized, decorticated Ascaris lumbricoides egg. A. lumbricoides is the largest nematode (roundworm) parasitizing the human intestine. From Public Health Image Library (PHIL).

• 

  Magnified 128X, this photomicrograph revealed some of the ultrastructural features displayed by a fertilized, decorticated Ascaris lumbricoides egg. A. lumbricoides is the largest nematode (roundworm) parasitizing the human intestine. From Public Health Image Library (PHIL).

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  Under a magnification of 125x, this photomicrograph of an unstained mounted formalin-preserved fecal sample revealed the presence of a number of parasitic worm eggs, which included the eggs of a trematode, Fasciolopsis buski, an Ascaris sp. nematode. From Public Health Image Library (PHIL).

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  Infertile egg of Ascaris lumbricoides. From Public Health Image Library (PHIL).

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  This photomicrograph revealed some of the ultrastructural features displayed by an infertile Ascaris lumbricoides egg. A. lumbricoides is the largest nematode (roundworm) parasitizing the human intestine. From Public Health Image Library (PHIL).

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  This micrograph reveals a fertilized egg of the round worm Ascaris lumbricoides; Mag. 400X. From Public Health Image Library (PHIL).

Epidemiology

More than 2 billion people are affected by this infection. The United States has a reported prevalence of 0.8% of the total population as of 1987. A. lumbricoides eggs are extremely resistant to strong chemicals, desiccation, and low temperatures. The eggs can remain viable in the soil for several months or even years.

Eggs of A. lumbricoides have been identified in archeological coprolites in the Americas, Europe, Africa, the Middle East, and New Zealand, the oldest ones being more than 24,000 years old.

Infections

Main article: Ascariasis

Infections with these parasites are more common where sanitation is poor, and raw human feces are used as fertilizer.

Symptoms

Often, no symptoms are seen with an A. lumbricoides infection. However, in the case of a particularly bad infection, symptoms may include bloody sputum, cough, fever, abdominal discomfort, intestinal ulcer, passing worms, etc. Ascariasis is also the most common cause of Löffler's syndrome worldwide. Accompanying symptoms include pulmonary infiltration, eosinophilia, and radiographic opacities

Prevention

Preventing any fecal-borne disease requires educated hygienic habits/culture and effective fecal treatment systems. This is particularly important with A. lumbricoides because its eggs are one of the most difficult pathogens to kill (second only to prions), and the eggs commonly survive 1–3 years. A. lumbricoides lives in the intestine where it lays eggs. Infection occurs when the eggs, too small to be seen by the unaided eye, are eaten. The eggs may get onto vegetables when improperly processed human feces of infected people are used as fertilizer for food crops. Infection may occur when food is handled without removing or killing the eggs on the hands, clothes, hair, raw vegetables/fruit, or cooked food that is (re)infected by handlers, containers, etc. Bleach does not readily kill A. lumbricoides eggs, but it will remove their sticky film, to allow the eggs to be rinsed away. A. lumbricoides eggs can be reduced by hot composting methods, but to completely kill them may require rubbing alcohol, iodine, specialized chemicals, cooking heat, or "unusually" hot composting (for example, over 50 °C (122 °F) for 24 hours)

Details of infection process

Infections happen when a human swallows water or food contaminated with unhatched eggs, which hatch into juveniles in the duodenum. They then penetrate the mucosa and submucosa and enter venules or lymphatics. Next, they pass through the right heart and into pulmonary circulation. They then break out of the capillaries and enter the air spaces. Acute tissue reaction occurs when several worms get lost during this migration and accumulate in other organs of the body. The juveniles migrate from the lung up the respiratory tract to the pharynx where they are swallowed. They begin producing eggs within 60–65 days of being swallowed. These are produced within the small intestine, where the juveniles mature. It might seem odd that the worms end up in the same place where they began. One hypothesis to account for this behavior is that the migration mimics an intermediate host, which would be required for juveniles of an ancestral form to develop to the third stage. Another possibility is that tissue migration enables faster growth and larger size, which increases reproductive capacity.

Diagnosis and treatment

Most diagnoses are made by identifying the appearance of the worm or eggs in feces. Due to the large quantity of eggs laid, physicians can diagnose using only one or two fecal smears.

Infections can be treated with drugs called ascaricides. The treatment of choice is mebendazole. The drug functions by binding to tubulin in the worms' intestinal cells and body-wall muscles. Nitazoxanide and ivermectin can also be used.

External links

• Ascaris lumbricoides Video - DAVE Project
• Ascaris lumbricoides Poll - Research
• Ascaris lumbricoides image library